1. Field of the Invention
The present invention relates to a multiplex transmission apparatus which is connected to multiplex transmission lines to detect signals transmitted to the multiplex transmission lines.
2. Description of the Related Art
This type of multiplex transmission apparatus is found, for example, in the publication of Unexamined Japanese Patent Application No. H2-20998. In this multiplex transmission apparatus, three comparators are connected to two transmission lines, i.e., the first and second transmission lines, via a filter circuit. These comparators compare voltages received from the two transmission lines, and also compare the voltage received from one of the transmission lines with a present bias voltage. Outputs from these comparators are supplied to a fault detecting means and a multiplexer. The fault detecting means detects faults in the transmission lines in accordance with outputs of these comparators and sends the first and second output signals based on the aforementioned fault to the multiplexer. In the multiplexer, one of the outputs of the comparators is selected, and the selected output is supplied to a multiplex transmission control circuit or the like. This multiplexer determines the above-mentioned output to be selected in accordance with output signals received from the aforementioned fault detecting means.
More specifically, if neither transmission line has a fault, then both output signals of the fault detecting means become "0", and the output of the first comparator, for instance, is selected as the output of the multiplexer. If there is a fault in the first transmission line, then the first output signal from the fault detecting means becomes "0", while the second output signal becomes "1", the output of the second comparator being selected as the selected output of the multiplexer. If the second transmission line has a fault, the first output signal from the fault detecting means becomes "1", and the second output signal "1", the output of the third comparator being selected as the selected output of the multiplexer. Thus, in the multiplex transmission apparatus, signals received from both transmission lines are supplied to three comparators, the fault detecting means detects faults in the transmission lines from the outputs of the comparators, and the multiplexer selects one receivable comparator output in accordance with the detection result.
In this multiplex transmission apparatus, if a fault takes place in a transmission line, comparator outputs are switched by the multiplexer even while signal messages are being received from the transmission line. This disables the multiplex transmission apparatus from receiving the messages and therefore the apparatus has to re-receive the same messages which are re-sent. Thus, in multiplex transmission which requires quick message transmission, this multiplex transmission apparatus is likely to develop delayed operation due to delay in transmission. Further, in this multiplex transmission apparatus, a large traffic volume of the transmission lines results in a further increased traffic volume, possibly leading to significant transmission delay in the whole multiplex transmission system. Further, the multiplex transmission apparatus disclosed in the aforementioned Japanese publication uses a special frame, and thus is limited in application.